The present invention generally relates to a device and method for measuring the physical property of a material as a function of that material's temperature, and more particularly to a device and method for measurement of solid-liquid and solid-state phase transformations and structural changes in materials under simulated and actual conditions associated with thermal and thermo-mechanical processing of such materials.
The solid-liquid and solid-state phase transformation and structural changes that occur in metallic materials during thermal and thermo-mechanical processing determine their final microstructure and hence their mechanical and physical (in-service) properties. These phase transformations and structural changes are also closely related to the fabricability (weldability, castability, formability or the like) of conventional and modern structural alloys, and consequently to their successful implementation as materials for advanced structural applications.
Extensive investigations have been performed worldwide, aimed at developing practical methods to measure and subsequently control the phase transformations and structural changes in metallic materials, in order to improve their fabricability and structural properties. Effective methods for studying of phase transformations and structural changes can be used for the development of new materials and advanced processing applications.
There are known various techniques and devices for evaluating phase transformation information in simulated and actual material processing environments. Examples of such evaluation include thermal, differential thermal and dilatometric analyses.
The method for thermal analysis (TA) allows direct determination of phase transformation temperatures from heating/cooling curves where large thermal effects cause significant change of the heating/cooling rate. This method is mainly applicable in equilibrium conditions and can be used in simulated, as well as in-situ, conditions. TA can also be applied during processing when the phase transformations are accompanied by release of large amounts of heat as in solidification of castings, or large weld pools. TA is insensitive to phase transformations and structural changes with small thermal effects that occur at non-equilibrium heating and cooling rates, as for example in welding.
Differential thermal analysis (DTA), is used for investigating phase transformations in materials where a specimen of the investigated material and a reference specimen experience the same heating and cooling thermal cycle in a controlled environment. In such technique, the material of the reference specimen does not undergo any phase changes in the investigated temperature range, while that of the specimen being investigated typically does. The heat consumption or release that accompany the phase transformations impact the heating and cooling rates of the investigated specimen. These heat effects are revealed by plotting the difference in the temperature of the two specimens versus time or the current temperature. Changes in the sample that lead to release or absorption of heat can be used in determining the phase transformation temperatures of the sample. The method and devices for DTA have high sensitivity to the thermal effects of phase transformations and structural changes. The traditional DTA technique utilizes sophisticated and expensive devices that operate in a short range of heating and cooling rates of up to about twenty degrees Celsius per minute. Such limitation makes this approach inapplicable for investigating the phase transformation in materials where actual processing conditions must be simulated. For example, the heating and the cooling rates of actual processing conditions are normally much higher, reaching up to several hundred degrees per second.
Continuous cooling transformation (CCT) diagrams are one of two main types of transformation diagrams that are used to optimize a metal's processing path to achieve a given set of properties. CCT diagrams measure the extent of phase transformation as a function of time for a continuously decreasing temperature. This allows the metal to be heated and then cooled at some rate so that the degree of transformation can be measured by dilatometry or other methods. In welding (for example) these diagrams allow the welding engineer to select the range of cooling rates, and the respective operational window of heat inputs that provides the optimal combination of microstructural constituents in the heat affected zone (HAZ) and weld metal. CCT diagrams are typically constructed by simulating weld thermal histories over numerous laboratory scale specimens. This approach is limited in depicting the actual heating and cooling rates and thermal gradients, and utilizes expensive, specialized equipment. Such an approach is not applicable for investigating solidification and solid-state phase transformations in the weld metal and therefore is not useful for constructing weld metal CCT diagrams. DTA methods and devices were also used for constructing CCT diagrams; however, because of the maximum heating and cooling rates for these techniques are so low, they are generally inappropriate for constructing a useful CCT diagram.
Methods for investigating the phase transformations under actual welding conditions have been conducted, where the temperature changes in a particular point of a real welded joint during welding were recorded. The method of differentiation of recorded thermal histories can be applied in-situ by analog or digital differentiation of the weld thermal history, in order to reveal the small thermal effects of phase transformations occurring in the HAZ. Disadvantages of such a method include the amplification of electromagnetic noise, recorded over the thermal cycle, the low sensitivity to and difficult recognition of higher temperature phase transformations, and low accuracy of determining phase transformation starting and finishing temperatures.
More recently, the original two-thermocouple version of DTA was applied during actual welding and partly solved some of the above mentioned problems. In such approach the reference thermal cycle is recorded by a thermocouple inserted into a tube of austenitic stainless steel, which does not undergo solid-state phase changes. The two thermocouples are equally positioned into the heat affected zone (HAZ) of the investigated carbon steel. The measured and the reference thermal histories, and the temperature difference between them are recorded. The sensitivity to heat effects of phase transformations in this approach depends on the distance between the two thermocouples, and further needs repetitive experiments to be optimized. In addition, the experiment is difficult to control. The manner of obtaining the reference thermal cycle limits the applicability of this approach only to solid-state phase transformations in the HAZ and does not allow investigating the solidification behavior and the other phase transformations in weld metal.
Dilatometric analysis (DA) is based on measuring the volume changes that accompany the phase transformations in metallic materials. This method is mainly applied in combination with devices for simulation of thermal and thermo-mechanical processing, and is capable of determining the solid-state phase transformations only. Since DA has low sensitivity to some solid-state phase transformations, it is inapplicable for solid-liquid phase transformations, and is insensitive to most structural changes. Dilatometry can be used to evaluate actual thermal cycles and heating and cooling rates associated with those cycles in such a way as to quantify dimensional changes of the material produced by such changes in temperature. Nevertheless, DA cannot be used in-situ, as it requires special sample types and can only detect phase transformations where there is a significant change in sample dimension. In addition, it cannot detect structural changes, such as recrystallization.
Different devices for simulation of thermal and thermo-mechanical processing are available. Such devices are based on resistance, induction, convection or light radiation heating of laboratory scale specimens. These devices use dilatometric analysis for determining the solid-state phase transformation temperatures. Because of the control loops used for controlling the heating and cooling rates, DTA is inapplicable in combination with such devices. Some thermo-mechanical simulators are not capable of reproducing the extremely high heating and cooling rates at the high temperature range that are typical for the most welding processes. This results in longer dwell times in austenite phase field (for steels), leading to larger grain size, lower transformation temperatures and consequently higher content of lower temperature products of austenite decompositions and higher hardness in the simulation specimens, compared to the real HAZ. The above technique is not capable of simulating the solid-liquid and solid-state phase transformations. In addition, the simulation equipment is complex in shape and involves expensive laboratory setups.
In another form, weld microstructure simulation equipment based on light radiation heating has been disclosed. The specimen is heated by focused high-power lamps and the specimen's temperature is controlled by a thermocouple. The cooling rate is controlled by the flow rate of a stream of shielding gas and simultaneous heating. The HAZ simulation specimen is a small thin wall tube that is in continuous contact with a dilatometer that is used to determine the phase transformation temperatures. The weld simulation specimen is a small cylinder that is attached to a thermocouple and melts over it forming a small ball. The solidification temperature range is determined by differentiation of the thermal history (measured by the thermocouple) using an analog electronic device. The main disadvantages of this equipment are that it has difficulty in resembling the actual weld and HAZ heating and cooling rates. In addition, it exhibits non-uniform heating of the HAZ simulation specimen. Moreover, the small volume of the weld simulation specimen does not allow resembling the actual weld solidification patterns.
The available methods and devices have a number of disadvantages that limit their usefulness for measuring phase transformations and structural changes in structural alloys during actual or simulated processing. Thus, what is desired is a method for determining phase transformation temperatures and structural changes under either simulated or actual operating environments for material processing. What is also desired is a device for performing more accurate simulations to evaluate material phase transformations and structural changes.